Human leukocyte antigens (HLA) are highly polymorphic proteins which play a key role in immunological recognition and are implicated in adverse immunological reactions to tissue transplants, the variation in ability to respond to antigenic stimuli, and genetic susceptibility to autoimmune disease. This project investigates the role of HLA disparity in adverse reactions to tissue transplants. Single amino acid differences between HLA molecules of a donor graft and recipient can elicit profound immunological responses. Unfortunately conventional HLA typing methods are not sensitive enough to detect this level of polymorphism. This project takes advantage of new methods to test the hypothesis that particular polymorphic residues in HLA proteins play an important role in the molecular events that underlie the functions described above. This study focuses on patients who express a common type, HLA-B44, which has been identified as a target of cytolytic T lymphocytes in patients experiencing graft rejection or graft- versus-host disease (GvHD). The sequences of HLA alleles of bone marrow recipients and their donors will be determined using solid phase automated nucleotide sequencing methods that have recently been developed in this laboratory. The nucleotide sequences will provide the first opportunity to accurately assess HLA disparity of each donor-recipient pair. The impact of HLA disparity will be investigated using in vitro and in vivo approaches. The frequency and specificity of cytotoxic T lymphocyte precursors (CTLp) which are mediators of graft rejection and GvHD will be determined. The relationships between HLA disparity, CTLp, and clinical events (e.g., rejection, GvHD, and survival) will be examined. The long-term objectives are to understand the molecular mechanisms of immunological recognition and to apply that knowledge to the development of effective therapeutic strategies to prevent or overcome life- threatening immunological problems such as GvHD. Observations from a human transplant model provide insights that are immediately applicable to bone marrow transplantation using donors other than HLA identical siblings. This is relevant to approximately 70% of patients who might benefit from bone marrow transplant but lack an HLA identical family donor.